HEMICIDARIS. 



Hemicidaris luciensis, d'Orbigny, PL III, fig. 6 a, h, c, d, e,f. 



Hemicidaeis Luciensis. D'Orbigny, Prodrome de Paleontologie, tome i, p. 320. 



— — Desor, Synopsis des Echinides Fossiles, p. 52. 



— CONFLUENS. Forbes, Memoirs of the Geological Survey, Decade V, description 



of pi. 5. Notes on Hemicidaris. 



Test sub-spheroidal, flat at the base, and depressed on the upper surface ; ambulacral 

 areas straight below, sinuous above, more especially in large adult individuals ; four pairs 

 of semi-tubercles, which increase gradually in size from the peristome upwards ; two 

 marginal rows of small tubercles perforate and crenulate, with intervening miliary granules 

 below, but imperforate and approximated above ; inter-arabulacral areas with from six to 

 seven pairs of primary tubercles ; apical disc prominent ; base flat ; mouth opening large ; 

 peristome with nearly equal-sized lobes. 



Dimensions. — a. Height, seven tenths of an inch ; transverse diameter, one inch and 

 three twentieths of an inch. «r 



B. Height, eleven twentieths of an inch ; transverse diameter, one 

 inch. 



c. Height, five tenths of an inch ; transverse diameter, eight tenths of 

 an inch. 



Description. — This Hemicidaris, it appears, was mistaken on the Continent for 

 Hemicidaris crenularis, Lamarck, of the Coral Rag, from which it has been with justice 

 separated by M. d^Orbigny ; it has, up to the present time, been overlooked as an English 

 urchin, the specimens hitherto found at Minchinhampton having been almost indeter- 

 minable. One specimen, however, collected from a band of clay in the Great Oolite, and 

 communicated by my friend Mr. Lycett, has enabled me to compare this form with a good 

 series of type specimens sent from Luc, Ranville, and Langrune, by MM. Michelin and 

 Deslongchamps ; so that the English specimens are found to occur in the same geological 

 horizon as the original French types. 



The test is thick and sub-spheroidal, with a flat base, and a depressed summit (PL HI, 

 fig, 6 c) ; the ambulacral areas are narrow, slightly sinuous in young shells, but much 

 more markedly so in large specimens ; their bases are a little expanded, to give space to 

 the four or five pairs of semi-tubercles (fig. 6 b), which gradually increase in size from the 

 peristome to the last pair ; the upper portions of the areas are provided with two marginal 

 rows of small tubercles (fig. 6 a), set tolerably wide apart, and alternating with each other; 

 two rows of close-set miliary granules occupy the middle of the area (fig. 6 d), and circlets 

 of the same surround the small marginal tubercles ; on the upper part of the areas two 

 rows of marginal granules alone fill up the entire areal space (fig. 6 c) ; the poriferous 



